Description (Adapted from Application): In the last twenty years, dengue fever (DF) and its more severe manifestations, dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS), have emerged as a global health problem. Over this period, the incidence of the illness has increased 100-fold and hyperendemic transmission now characterizes most tropical countries, including Puerto Rico (PR). To understand the relationship between the rise of epidemic DF and DHIF/DSS and viral genotype diversity, genetic change will be examined in three dengue serotypes (DEN-1, DEN-2, and DEN-4) in Puerto Rico across an 18-year period. This time period traces the rise of DF and DHF/DSS on the island beginning with initial disease establishment and extending through repeated epidemic cycles. The investigators will sample serotype variation at two temporal scales--across multiple endemic and epidemic cycles, and at time points within single epidemic cycles. Nucleotide substitutions in the full complement of viral structural genes and two rapidly evolving non-structural genes will be used to construct a longitudinal series of phylogenetic trees for the DEN-1, DEN-2, and DEN-4 serotypes. These phylogenies will provide a temporal framework for tracing the history of the dengue epidemic in Puerto Rico. This framework will allow the investigators to: (1) document genotypic and evolutionary change over a period characterized by endemic and epidemic disease transmission; (2) correlate population genetic changes within and between dengue serotypes with cycles of endemic and epidemic transmission; (3) explore the nature of selection acting on different viral gene regions; (4) identify potentially significant amino acid positions and determine their association with disease severity and/or transmission potential; and (5) evaluate the importance of recombination in the evolution of the dengue virus in PR. This project is a synergistic collaboration linking molecular evolutionary biologists at the University of Puerto Rico with virologists at the Centers for Disease Control and Prevention (CDC). The study utilizes the CDC's rich viral sample bank and is the first systematic molecular epidemiological study of this emerging pandemic.